1. Field of Invention
This invention pertains generally to angular rate sensors and the like and, more particularly, to a highly reliable sensor system and method for use in applications such as ones where safety is critical.
2. Related Art
In a vibratory rate sensor or gyroscope, a mass is driven to vibrate or oscillate along a drive axis. Rotation of the sensor about an axis perpendicular to the drive axis causes a Coriolis force to be applied to the mass along a response axis which is perpendicular to the drive and sensing axes. The force is proportional to the product of the rate of rotation and the velocity of vibration, and the rate of rotation is determined by monitoring the force or the movement of the mass along the response axis.
Different types of sensing elements are used in such devices. Some are fabricated from silicon wafers, and others are fabricated of crystalline quartz and other piezoelectric materials.
With silicon sensing elements, the masses are commonly driven electrostatically, and the Coriolis induced forces are monitored capacitively. Such structures are generally planar, which tends to maximize the capacitance of the sensing elements.
Piezoelectric rate sensors are commonly in the form of tuning forks having at least one pair of tines which are positioned side-by-side and driven out of phase with each other in the plane of the tines. When the tuning fork is rotated about an axis parallel to the tines, the Coriolis force produces a second (pickup) mode of oscillation in which the tines vibrate in an antiphase manner perpendicular to the plane of the tines. Examples of such rate sensors are found in U.S. Pat. Nos. 4,654,663, 4,899,587, 5,396,144, 5,408,876, 5,585,561 and 6,262,520.
The tuning forks in such rate sensors often have more than one pair of tines, e.g. two pairs of tines arranged in an H-shaped configuration, with one pair being driven in the plane of the fork. The out-of-plane vibration produced by the Coriolis force is torsionally coupled to the other pair of tines, and the two pairs vibrate out-of-plane in opposite directions in the pickup mode. With a central mounting point, the out-of-phase motion of the two sets of tines cancels pickup mode forces at the mounting point, minimizing the effect of boundary conditions at the mount on the pickup mode oscillation.
In safety-critical applications such as automobile stability controls, built-in fault detection is also highly desirable. This typically involves a series of internal monitors to verify proper functioning of individual circuits within the sensor electronics and may include the application of a test signal to the sensing element to validate the integrity of the element and the path of its output signal. Examples of such sensors are found in U.S. Pat. Nos. 5,426,970 and 6,497,146.
Redundancy of sensors has also been utilized to increase reliability in critical applications, and an example of such as system is found in U.S. Pat. No. 6,462,530.
In some prior art sensors with analog outputs, a failure in the sensor is indicated by switching the output voltage to a predetermined level such as the positive rail. In sensors with digital outputs, a microprocessor or micro-controller monitors critical signal levels and indicates the detection of a failure by delivering an error code to the digital output. The digital approach has the advantage that the nature of the failure can be indicated by the error code. The error code can be output in parallel with the sensor data, giving the end user the option of how and/or whether to use the data.
If a digital sensor interface is bidirectional, it may be possible to apply commands to the sensor logic or micro-controller to reset any error flags which have been set.
Communication of the built-in test information depends upon the integrity of the digital interface. If the interface itself were to suffer a fault, the internal logic and/or micro-controller would be unable to transmit either the sensor data or fault diagnostic information. That is an unacceptable risk in applications where safety is critical.